The present invention relates to a new and improved method for the detection or recognition of a source generating electro-magnetic oscillations or energy and an apparatus for carrying out the aforesaid method.
Unfortunately, attempts are repeatedly undertaken by unauthorized persons by using a small transmitter, so-called miniature spy devices, to obtain illegal knowledge of conversations undertaken between individuals on a private or confidential basis. Oftentimes these miniature spy devices are transmitters which of necessity can only transmit a very small output, the range of which extends from a few meters up to several kilometers. The detection of such miniature transmitters by the eye is almost impossible within any useful period of time since the dimensions of such small transmitters, owing to the modern miniaturization techniques which are available, are so small that it is readily possible to construct the external form of such transmitters similar to objects which are in general use and conventionally employed, so that the possibility of effectively camouflaging these illegal miniature transmitters is extremely great.
It has already been proposed to search for the possibly concealed miniature transmitter by means of a wide band receiver having a search antenna, in that there is searched the area where one suspects there is located the miniature transmitter. In the event that there are not present any or only weak electro-magnetic fields emanating from other legal transmitters, for instance radio transmitters, then it is possible with such type receiver to find the hidden miniature transmitter with relatively little expenditure. However, if there are present electrical fields which emanate from legal transmitters -- which is normally the case -- then the sensitivity of the aforementioned receiver must be so markedly reduced that locating the illegal transmitter is rendered considerably more difficult.
Furthermore, there is known in this particular field of technology the so-called feedback technique. In this case there is employed a receiver having a loud speaker, this receiver being tuned over the frequency range which is to be controlled. If the frequency set at the receiver coincides with that of the miniature transmitter which is to be located, than there appears via the loud speaker of the receiver and the listening microphone of the miniature transmitter an acoustical feedback which is perceivable in the form of a whistling tone. Since the receiver employed with this technique is very selective, it can possess really good sensitivity. Yet, a considerable drawback of this technique resides in the fact that there is required an active cooperative action of the miniature transmitter which is to be located. This cooperative action can be, however, prevented at the miniature transmitter by carrying out certain circuit designs so as to foil this type of detection, with the result that also this technique has produced rather unsatisfactory results.